908 T. G. BONNEY OX THE SERPENTINE AND 



(sometimes rendering nearly the whole opaque), or clouded granules 

 disseminated in various ways. At last when the whole grain is 

 converted into serpentine, the black strings marking the original 

 cracks become less definite, being interrupted and disturbed, pro- 

 bably from the action of molecular forces, so that their significance 

 might readily be overlooked. The chrysotile strings also become less 

 conspicuous ; but still they may often be traced when the prisms 

 are crossed. It is worth noting that, as a rule, the formation of 

 chrysotile appears to proceed from the surface of a crack inwards, 

 and is generally arrested at a fairly constant distance from the out- 

 side ; but the conversion of the remainder of the grain into non- 

 crystalline serpentine appears to be nearly simultaneous over the 

 whole. The process very closely corresponds with that which I have 

 already described in the Ariege lherzolitc* (see fig. 9, p. 916), except 

 that here, as the olivine is apparently ferruginous, there is a more 

 marked discoloration. The augitic constituent is less abundant than 

 the other two minerals. Most of it is diallage ; but grains, and some- 

 times even parts of a crystal, seem rather to be normal augite. 

 Occasionally the serpentine appears to have been deposited in a crack 

 in them : but as a rule there is not the slightest advance towards a 

 conversion of these minerals into serpentine. 



The diallage occasionally assumes a dusky, finely granulated, 

 stained aspect along the principal cleavage-planes, which I have 

 observed in cases where a change to hornblende is commencing ; and 

 one crystal is bordered by an aggregate of minute crystals, which I 

 fully believe to be actinolite. There are few included microliths, 

 excluding these ferruginous clots ; but there seem to be one or two 

 grains of magnetite, and in one of the diallage crystals are two 

 needles which I think are apatite. 



Slide II. (cut from a rather duller-coloured specimen) does 

 not materially differ, except that the iron stains in the olivine 

 are more uniformly dark. Some of the diallage (which mineral is 

 fairly abundant in this specimen) contains small opaque belonitic 

 microliths lying parallel to the plane of principal cleavage. 



In slide III. the felspar decidedly predominates over the other mi- 

 nerals, the olivine coming next. Some of the grains here are wholly 

 converted into serpentine ; one shows, with polarized light, a single 

 speck of the original mineral alone remaining in a good-sized grain. 

 There are both diallage and augite ; and here, as in the other slides, 

 there are appearances of a passage from one variety to the other. One 

 crystal shows in parts the ordinary imperfect cleavages of augite parallel 

 to ocP, with an occasionally marked clino-diagonal cleavage ; then, 

 in places, the last cleavage dominates over the other, forming very 

 definite parallel lines of weak cohesion ; and, lastly, the inner (and 

 major) portion of the crystal is filled by a fine parallel cleavage, 

 giving it an almost granular-linear aspect, in which, I think, 

 occasional cracks indicate traces of a cleavage parallel to ocP. The 

 absorptive powers of the two minerals seem to differ. By rotating 



* Gcol. Mag. Dec. 2, vol. iv. p. 59. 



